Semiconductor device

ABSTRACT

A semiconductor device capable of high-speed operation. The semiconductor device includes a first transistor, a second transistor, and a capacitor. One of a source and a drain of the first transistor is supplied with a first signal. One of a source and a drain of the second transistor is supplied with a first potential. A gate of the second transistor is supplied with a second signal. A first electrode of the capacitor is electrically connected to the other of the source and the drain of the first transistor. A second electrode of the capacitor is electrically connected to the other of the source and the drain of the second transistor. In a first period, the first signal is low and the second signal is high. In a second period, the first signal is high and the second signal is either low or high.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to semiconductordevices, display devices, and the like.

2. Description of the Related Art

The increase in size of display devices such as liquid crystal displaydevices and EL display devices promotes the development of displaydevices with higher added value. In particular, techniques by which adriver circuit in a display device is composed of only transistorshaving the same conductivity type have been actively developed (seePatent Document 1).

FIG. 16 illustrates a driver circuit disclosed in Patent Document 1. Thedriver circuit in Patent Document 1 includes transistors M1 to M7. Whena signal GOUT[N−1] goes high, the transistor M3 is turned on. Thus, avoltage VON is supplied to a gate of the transistor M1, so that the gatepotential of the transistor M1 starts to rise. Since the gate potentialof the transistor M1 rises gradually, a potential difference between agate and source (hereinafter also referred to as Vgs) of the transistorM3 decreases gradually. Then, Vgs of the transistor M3 becomes thethreshold voltage of the transistor M3, and the transistor M3 is turnedoff.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2004-103226

SUMMARY OF THE INVENTION

In the driver circuit in Patent Document 1, Vgs of the transistor M3decreases gradually as the gate potential of the transistor M1 rises.That is, the drain current of the transistor M3 decreases gradually. Asa result, the time after the signal GOUT[N−1] goes high until thetransistor M3 is turned off is long. In addition, the gate of thetransistor M1 needs to be brought into a floating state by turning offthe transistor M3 before a signal CKV goes high. Consequently, it isdifficult to increase the driving frequency of the driver circuit inPatent Document 1.

Further, in the driver circuit in Patent Document 1, the ratio ofchannel width (W) to channel length (L) (hereinafter referred to as W/L)of the transistor M3 needs to be increased in order to raise the gatepotential of the transistor M1 rapidly. Thus, the size of the transistorM3 increases, resulting in the increase in layout area.

In the driver circuit in Patent Document 1, the gate of the transistorM1 is required to be floating. However, charge is leaked from the gateof the transistor M1 because of the off-state current of a transistorwhose source or drain is connected to the gate of the transistor M1. Itis therefore difficult to prolong a period during which the gate of thetransistor M1 is floating. That is, it is difficult to lower the drivingfrequency.

As described above, it is difficult to both increase and lower thedriving frequency of the driver circuit in Patent Document 1, so thatthe range of the driving frequencies at which the driver circuitoperates normally is narrow.

In view of the above, an object of one embodiment of the presentinvention is to provide a driver circuit that can operate even at highdriving frequencies. Another object is to provide a driver circuit thatcan operate even at low driving frequencies. Another object is toprovide a driver circuit that can operate at a wide range of drivingfrequencies. Another object is to decrease W/L of a transistor. Stillanother object is to provide a circuit having a novel configuration.Note that an object and an effect are inseparable, and it is apparentthat an effect described in this specification and the like isaccompanied by an object associated with the effect. On the other hand,it is apparent that an object described in this specification and thelike is accompanied by an effect associated with the object.

One embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, and a capacitor. Oneof a source and a drain of the first transistor is supplied with a firstsignal. One of a source and a drain of the second transistor is suppliedwith a first potential. A gate of the second transistor is supplied witha second signal. A first electrode of the capacitor is electricallyconnected to the other of the source and the drain of the firsttransistor. A second electrode of the capacitor is electricallyconnected to the other of the source and the drain of the secondtransistor. In a first period, the first signal is low and the secondsignal is high. In a second period, the first signal is high and thesecond signal is either low or high.

Another embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, a third transistor,and a capacitor. One of a source and a drain of the first transistor issupplied with a first signal. One of a source and a drain of the secondtransistor is supplied with a first potential. A gate of the secondtransistor is supplied with a second signal. One of a source and a drainof the third transistor is supplied with the first signal. A gate of thethird transistor is electrically connected to a gate of the firsttransistor. A first electrode of the capacitor is electrically connectedto the other of the source and the drain of the first transistor. Asecond electrode of the capacitor is electrically connected to the otherof the source and the drain of the second transistor. In a first period,the first signal is low and the second signal is high. In a secondperiod, the first signal is high and the second signal is either low orhigh.

In any of the above embodiments of the present invention, thesemiconductor device may also include a fourth transistor, a fifthtransistor, and a sixth transistor. One of a source and a drain of thefourth transistor is supplied with the first potential. The other of thesource and the drain of the fourth transistor is electrically connectedto the other of the source and the drain of the first transistor. A gateof the fourth transistor is electrically connected to the other of thesource and the drain of the second transistor. One of a source and adrain of the fifth transistor is supplied with the first potential. Theother of the source and the drain of the fifth transistor iselectrically connected to the gate of the first transistor. A gate ofthe fifth transistor is electrically connected to the other of thesource and the drain of the second transistor. One of a source and adrain of the sixth transistor is supplied with a second potential. Theother of the source and the drain of the sixth transistor iselectrically connected to the other of the source and the drain of thesecond transistor. A gate of the sixth transistor is supplied with athird signal.

In any of the above embodiments of the present invention, W/L of thefirst transistor may be higher than that of the second transistor.

Embodiments of the present invention can provide a driver circuit thatcan operate even at high driving frequencies, provide a driver circuitthat can operate even at low driving frequencies, provide a drivercircuit that can operate at a wide range of driving frequencies, anddecrease W/L of a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams for explaining a basic circuit according toone embodiment of the present invention;

FIGS. 2A to 2C are diagrams for explaining a basic circuit according toone embodiment of the present invention;

FIG. 3 illustrates a sequential circuit according to one embodiment ofthe present invention;

FIG. 4 is a diagram for explaining a sequential circuit according to oneembodiment of the present invention;

FIG. 5 illustrates a shift register circuit according to one embodimentof the present invention;

FIGS. 6A and 6B illustrate a basic circuit and a sequential circuit,respectively, according to one embodiment of the present invention;

FIGS. 7A and 7B each illustrate a sequential circuit according to oneembodiment of the present invention;

FIGS. 8A and 8B each illustrate a sequential circuit according to oneembodiment of the present invention;

FIGS. 9A and 9B each illustrate a sequential circuit according to oneembodiment of the present invention;

FIGS. 10A and 10B each illustrate a sequential circuit according to oneembodiment of the present invention;

FIGS. 11A to 11E are diagrams for explaining a sequential circuitaccording to one embodiment of the present invention;

FIG. 12 is a diagram for explaining a display device according to oneembodiment of the present invention;

FIGS. 13A to 13D each illustrate a transistor according to oneembodiment of the present invention;

FIG. 14 illustrates a display device according to one embodiment of thepresent invention;

FIGS. 15A to 15E each illustrate an electronic device according to oneembodiment of the present invention; and

FIG. 16 illustrates a conventional driver circuit.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described belowwith reference to the accompanying drawings. Note that it will bereadily appreciated by those skilled in the art that details of theembodiments can be modified without departing from the spirit and scopeof the present invention. The present invention is therefore not limitedto the following description of the embodiments.

Note that the present invention includes, in its category, anysemiconductor device including transistors, for example, integratedcircuits, RF tags, and display devices. The integrated circuit includes,in its category, large scale integrated circuits (LSIs) including amicroprocessor, an image processing circuit, a digital signal processor(DSP), a microcontroller, and the like and programmable logic devices(PLDs) such as a field programmable gate array (FPGA) and a complex PLD(CPLD). The display device includes, in its category, a liquid crystaldisplay device, a light-emitting device in which a light-emittingelement typified by an organic light-emitting element (OLED) is providedin each pixel, electronic paper, a digital micromirror device (DMD), aplasma display panel (PDP), a field emission display (FED), and thelike.

In this specification, the display device includes, in its category, apanel in which a display element such as a liquid crystal element or alight-emitting element is provided in each pixel, and a module in whichan IC or the like including a controller is mounted on the panel.

Note that in this specification, the term “connection” means electricalconnection and corresponds to a state in which current, voltage, or apotential can be supplied or transmitted. Therefore, a state of being“connected” means not only a state of direct connection but also a stateof indirect connection through an element such as a wiring, a resistor,a diode, or a transistor, in which current, voltage, or a potential canbe supplied or transmitted. In addition, even when independentcomponents are connected to each other in a circuit diagram, there isactually a case where one conductive film has functions of a pluralityof components, such as the case where part of a wiring functions as anelectrode. The term “connection” in this specification also means such acase where one conductive film has functions of a plurality ofcomponents.

Note that a “source” of a transistor means a source region that is partof a semiconductor film functioning as an active layer or a sourceelectrode connected to the semiconductor film. Similarly, a “drain” of atransistor means a drain region that is part of the semiconductor filmor a drain electrode connected to the semiconductor film. A “gate” meansa gate electrode.

The terms “source” and “drain” of a transistor interchange with eachother depending on the polarity of the transistor or the levels ofpotentials applied to the terminals. In general, in an n-channeltransistor, a terminal to which a lower potential is applied is called asource, and a terminal to which a higher potential is applied is calleda drain. Further, in a p-channel transistor, a terminal to which a lowerpotential is applied is called a drain, and a terminal to which a higherpotential is applied is called a source. In this specification, forconvenience, connection relation of the transistor is sometimesdescribed assuming that the source and the drain are fixed; actually,the names of the source and the drain interchange with each otherdepending on the relation of the potentials.

Embodiment 1

In this embodiment, a basic circuit, a sequential circuit including thebasic circuit, and a shift register circuit including the sequentialcircuit which are embodiments of the present invention will bedescribed.

First, the configuration of a basic circuit in this embodiment will bedescribed with reference to FIG. 1A.

The basic circuit in FIG. 1A includes a transistor 101, a transistor102, and a capacitor 110.

A first terminal of the transistor 101 is connected to a wiring 11. Asecond terminal of the transistor 101 is connected to a wiring 12.

A first terminal of the transistor 102 is connected to a wiring 13. Agate of the transistor 102 is connected to a wiring 14.

A first electrode (also referred to as one electrode) of the capacitor110 is connected to the wiring 12. A second electrode (also referred toas the other electrode) of the capacitor 110 is connected to a secondterminal of the transistor 102.

Note that the second terminal of the transistor 102 or the secondelectrode of the capacitor 110 is indicated by a node N1. A gate of thetransistor 101 is indicated by a node N2.

Note that the transistor 101 and the transistor 102 preferably have thesame conductivity type. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

Note that in this specification and the like, the term “connection”means electrical connection and corresponds to a state in which current,voltage, a potential, a signal, charge, or the like can be supplied ortransmitted. The state of being “connected” therefore means not only astate of direct connection but also a state of indirect connectionthrough an element such as a wiring, a conductive film, a resistor, adiode, a transistor, or a switching element, for example.

A first terminal of a transistor is also referred to as one of a sourceand a drain of the transistor or a first electrode of the transistor. Asecond terminal of the transistor is also referred to as the other ofthe source and the drain of the transistor or a second electrode of thetransistor.

The wiring 11 (also referred to as signal line) is supplied with asignal CK and has a function of transmitting or supplying the signal CK.The signal CK is a signal having high and low levels. The signal CKcorresponds to one of a plurality of clock signals input to a shiftregister circuit.

The wiring 12 (also referred to as signal line) outputs a signal OUT andhas a function of transmitting or supplying the signal OUT. The signalOUT is a signal having high and low levels. Moreover, the signal OUT isan output signal of the basic circuit illustrated in FIG. 1A. The signalOUT corresponds to one of a plurality of output signals output from ashift register circuit or an output signal of a sequential circuit.

The wiring 13 (also referred to as power supply line) is supplied with apotential VSS (also referred to as first potential) and has a functionof transmitting or supplying the potential VSS. The potential VSS is aconstant potential.

The wiring 14 (also referred to as signal line) is supplied with asignal SP and has a function of transmitting or supplying the signal SP.The signal SP is a signal having high and low levels. Further, thesignal SP is a signal for controlling the on/off state of the transistor102. The signal SP corresponds to, for example, a start pulse input to ashift register circuit or an output signal of the preceding sequentialcircuit or any sequential circuit before the preceding sequentialcircuit.

The transistor 101 has a function of controlling electrical continuitybetween the wiring 11 and the wiring 12, a function of supplying thesignal CK of the wiring 11 to the wiring 12, and a function ofmaintaining a potential difference between the wiring 12 and the nodeN2.

The transistor 102 has a function of controlling electrical continuitybetween the wiring 13 and the node N1, and a function of supplying thepotential VSS of the wiring 13 to the node N1.

The capacitor 110 has a function of maintaining a potential differencebetween the wiring 12 and the node N1.

Next, a method for driving the basic circuit in FIG. 1A will bedescribed with reference to a timing chart in FIG. 1B and FIGS. 2A to2C.

Here, the description is made assuming that a high-level potential ofthe signal CK and the signal SP is a potential VDD (also referred to assecond potential) and a low-level potential thereof is the potentialVSS. Note that the potential VDD is higher than the potential VSS.

Furthermore, the description is made on the assumption that an initialpotential of the node N1 is the potential VDD, an initial potential ofthe node N2 is the potential VSS, and an initial potential of the wiring12 is the potential VSS. Note that when the initial potential of thenode N2 is the potential VSS, the transistor 101 is off in the initialstate.

For convenience of description, a period necessary for the operation isdivided into a period Ta and a period Tb.

First, in the period Ta, the signal SP goes high and the signal CK goeslow. When the signal SP goes high, the transistor 102 is turned on. Whenthe transistor 102 is turned on, the potential VSS of the wiring 13 issupplied to the node N1. Thus, the potential of the node N1 decreases tothe potential VSS. At this time, the capacitor 110 maintains a potentialdifference between the node N1 and the wiring 12 and the transistor 101is off; consequently, the wiring 12 is floating. Thus, the potential ofthe wiring 12 decreases from the potential VSS as the potential of thenode N1 decreases. Then, the transistor 101 is turned on when thepotential of the wiring 12 becomes lower than a potential obtained bysubtracting the threshold voltage of the transistor 101 from thepotential (e.g., the potential VSS) of the node N2 (see FIG. 2A).

When the transistor 101 is turned on, the signal CK of the wiring 11 issupplied to the wiring 12. Since the signal CK is low, the potential ofthe wiring 12 increases to the potential VSS. At this time, thetransistor 101 maintains a potential difference between the node N2 andthe wiring 12, and the node N2 is floating. Thus, the potential of thenode N2 rises as the potential of the wiring 12 rises. When thepotential of the node N2 exceeds a potential obtained by adding thepotential (e.g., the potential VSS) of the wiring 11 and the thresholdvoltage of the transistor 101, the transistor 101 is kept on.Consequently, the potential of the wiring 12 increases to the potentialVSS. That is, the signal OUT becomes low (see FIG. 2B).

Then, in the period Tb, the signal SP goes low and the signal CK goeshigh. When the signal SP goes low, the transistor 102 is turned off. Thetransistor 101 remains on as described above, so that the signal CK ofthe wiring 11 continues to be supplied to the wiring 12. Since thesignal CK is high, the potential of the wiring 12 increases from thepotential VSS. At this time, the transistor 101 maintains a potentialdifference between the node N2 and the wiring 12, and the node N2 iskept floating. Consequently, the potential of the node N2 rises as thepotential of the wiring 12 rises. When the potential of the node N2exceeds a potential obtained by adding the potential (e.g., thepotential VDD) of the wiring 11 and the threshold voltage of thetransistor 101, the transistor 101 is kept on. Thus, the potential ofthe wiring 12 increases to the potential VDD. That is, the signal OUTbecomes high (see FIG. 2C).

In the basic circuit in FIG. 1A, Vgs of the transistor 102 can be kepthigh in the period Ta, so that the drain current of the transistor 102can be kept high. Thus, the potential of the node N1 can be rapidlydecreased, whereby the period Ta can be shortened. In other words, thedriving frequency can be increased.

Keeping Vgs of the transistor 102 high leads to the decrease in W/L ofthe transistor 102. Thus, advantages such as reduction in the layoutarea and decrease in the input capacitance are obtained.

A capacitor may be connected between the gate and the second terminal ofthe transistor 101, in which case the capacitance between the node N2and the wiring 12 can be increased and as a result, the potential of thenode N2 can be made higher.

In the period Tb, the signal SP may be kept high. In this case, thetransistor 102 is kept on in the period Tb, so that the potential VSS ofthe wiring 13 continues to be supplied to the node N1. Consequently, thechange in the potential of the node N1 associated with the change in thepotential of the wiring 12 can be prevented.

Alternatively, in the period Tb, the signal SP may be kept high as inthe period Ta and then become low. In this case, the transistor 102remains on and then is turned off in the period Tb, so that thepotential VSS of the wiring 13 continues to be supplied to the node N1in a period during which the potential of the wiring 12 is changedwithin the period Tb. Thus, the change in the potential of the node N1associated with the change in the potential of the wiring 12 can beprevented.

In the case where a load is connected to the wiring 12, the load isdriven by the transistor 101. Therefore, W/L of the transistor 101 ispreferably higher than that of the transistor 102.

A transistor may be used as the capacitor 110, in which case it ispreferable that a gate of the transistor be connected to the node N1 anda first terminal and/or a second terminal of the transistor be connectedto the wiring 12. In other words, the capacitor 110 may include asemiconductor layer connected to the wiring 12, a gate electrodeconnected to the node N1, and a gate insulating layer between thesemiconductor layer and the gate electrode. Consequently, thecapacitance between the node N1 and the wiring 12 can be large when thepotential of the node N1 decreases in the period Ta.

The transistor 102 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the node N1.

Note that the high-level potential of the signal SP may be lower thanthe potential VDD, in which case the amplitude voltage of the signal SPcan be reduced to cut power consumption.

The low-level potential of the signal SP may be lower than the potentialVSS, in which case the transistor 102 can be definitely turned off evenif the transistor 102 is a normally-on transistor.

Next, a sequential circuit including the basic circuit in FIG. 1A willbe described.

First, the configuration of a sequential circuit in this embodiment willbe described with reference to FIG. 3. The sequential circuit in FIG. 3includes transistors 103 to 105 in addition to the basic circuitillustrated in FIG. 1A.

Note that the transistors 101 to 105 preferably have the sameconductivity type. In this embodiment, the case where these transistorsare n-channel transistors will be described.

A first terminal of the transistor 103 is connected to the wiring 13. Asecond terminal of the transistor 103 is connected to the wiring 12. Agate of the transistor 103 is connected to the node N1.

A first terminal of the transistor 104 is connected to the wiring 13. Asecond terminal of the transistor 104 is connected to the node N2. Agate of the transistor 104 is connected to the node N1.

A first terminal of the transistor 105 is connected to a wiring 15. Asecond terminal of the transistor 105 is connected to the node N1. Agate of the transistor 105 is connected to a wiring 16.

The wiring 15 (also referred to as power supply line) is supplied withthe potential VDD and has a function of transmitting or supplying thepotential VDD.

The wiring 16 (also referred to as signal line) is supplied with asignal RE and has a function of transmitting or supplying the signal RE.The signal RE is a signal having high and low levels. Further, thesignal RE is a signal for controlling the on/off state of the transistor105. The signal RE corresponds to, for example, a reset pulse input to ashift register circuit or an output signal of the following sequentialcircuit or any sequential circuit after the following sequentialcircuit.

The transistor 103 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 12, and a function of supplying thepotential VSS of the wiring 13 to the wiring 12.

The transistor 104 has a function of controlling electrical continuitybetween the wiring 13 and the node N2, and a function of supplying thepotential VSS of the wiring 13 to the node N2.

The transistor 105 has a function of controlling electrical continuitybetween the wiring 15 and the node N1, and a function of supplying thepotential VDD of the wiring 15 to the node N1.

Next, the operation of the sequential circuit in FIG. 3 will bedescribed with reference to a timing chart in FIG. 4.

The following description is made assuming that a high-level potentialof the signal CK, the signal SP, and the signal RE is the potential VDDand a low-level potential thereof is the potential VSS.

Furthermore, the description is made on the assumption that an initialpotential of the node N1 is the potential VDD, an initial potential ofthe node N2 is the potential VSS, and an initial potential of the wiring12 is the potential VSS. Since the initial potential of the node N2 isthe potential VSS, the transistor 101 is off in the initial state.

For convenience of description, a period necessary for the operation isdivided into periods Ta, Tb, Tc, and Td.

First, in the period Ta, the signal SP goes high, the signal RE goeslow, and the signal CK goes low. When the signal RE goes low, thetransistor 105 is turned off. When the signal SP goes high, thetransistor 102 is turned on. When the transistor 102 is turned on, thepotential VSS of the wiring 13 is supplied to the node N1. Thus, thepotential of the node N1 decreases to the potential VSS. When thepotential of the node N1 decreases, the transistors 103 and 104 areturned off. When the transistor 104 is turned off, the node N2 becomesfloating. Thus, the potential of the node N2 remains at the potentialVSS, so that the transistor 101 is kept off.

In a period during which the potential of the node N1 is decreasing, thecapacitor 110 maintains a potential difference between the node N1 andthe wiring 12, and the wiring 12 is floating since the transistors 101and 103 are off. Thus, the potential of the wiring 12 decreases from thepotential VSS as the potential of the node N1 decreases. The transistor101 is turned on when the potential of the wiring 12 becomes lower thana potential obtained by subtracting the threshold voltage of thetransistor 101 from the potential (e.g., the potential VSS) of the nodeN2. When the transistor 101 is turned on, the signal CK of the wiring 11is supplied to the wiring 12. Since the signal CK is low, the potentialof the wiring 12 increases. At this time, the transistor 101 maintains apotential difference between the node N2 and the wiring 12, and the nodeN2 is floating since the transistor 104 is off. Consequently, thepotential of the node N2 rises as the potential of the wiring 12 rises.When the potential of the node N2 exceeds a potential obtained by addingthe potential (e.g., the potential VSS) of the wiring 11 and thethreshold voltage of the transistor 101, the transistor 101 is kept on.Thus, the potential of the wiring 12 increases to the potential VSS.That is, the signal OUT becomes low.

Then, in the period Tb, the signal SP goes low, the signal RE is keptlow, and the signal CK goes high. Since the signal RE is kept low, thetransistor 105 is kept off. The transistor 102 is turned off because thesignal SP goes low. Thus, the node N1 becomes floating and the potentialof the node N1 is kept at the potential in the period Ta, so that thetransistors 103 and 104 remain off.

Here, since the transistor 101 is kept on, the signal CK of the wiring11 continues to be supplied to the wiring 12. Since the signal CK ishigh, the potential of the wiring 11 rises. At this time, the transistor101 maintains a potential difference between the node N2 and the wiring12, and the node N2 is floating since the transistor 104 is off.Consequently, the potential of the node N2 rises as the potential of thewiring 12 rises. When the potential of the node N2 exceeds a potentialobtained by adding the potential (e.g., the potential VDD) of the wiring11 and the threshold voltage of the transistor 101, the transistor 101is kept on. Thus, the potential of the wiring 12 increases to thepotential VDD. That is, the signal OUT becomes high.

Then, in the period Tc, the signal SP is kept low, the signal RE goeshigh, and the signal CK goes low. The transistor 102 is kept off becausethe signal SP is kept low. The transistor 105 is turned on because thesignal RE goes high. When the transistor 105 is turned on, the potentialVDD of the wiring 15 is supplied to the node N1, so that the potentialof the node N1 rises. Then, the transistor 105 is turned off when thepotential of the node N1 increases to a potential obtained bysubtracting the threshold voltage of the transistor 105 from the gatepotential (e.g., the potential VDD) of the transistor 105. Thus, thenode N1 becomes floating, and the potential of the node N1 remains high.When the potential of the node N1 increases, the transistors 103 and 104are turned on. When the transistor 104 is turned on, the potential VSSof the wiring 13 is supplied to the node N2. Thus, the potential of thenode N2 decreases to the potential VSS. When the potential of the nodeN2 decreases to the potential VSS, the transistor 101 is turned off.Further, when the transistor 103 is turned on, the potential VSS of thewiring 13 is supplied to the wiring 12. Thus, the potential of thewiring 12 decreases to the potential VSS. That is, the signal OUTbecomes low.

Then, in the period Td, the signal SP is kept low, the signal RE goeslow, and the signal CK oscillates between high and low levels. Since thesignal SP is kept low, the transistor 102 is kept off. Since the signalRE goes low, the transistor 105 is kept off. The node N1 is keptfloating because the transistors 102 and 105 remain off. Accordingly,the potential of the node N1 is kept at the potential in the period Tc,so that the transistors 103 and 104 are kept on. The potential VSS ofthe wiring 13 continues to be supplied to the node N2 while thetransistor 104 is kept on. Thus, the potential of the node N2 remains atthe potential VSS, and the transistor 101 remains off. Further, thepotential VSS of the wiring 13 continues to be supplied to the wiring 12while the transistor 103 is kept on. Thus, the potential of the wiring12 remains at the potential VSS. That is, the signal OUT remains low.

In the sequential circuit in FIG. 3, the potential of the wiring 12becomes lower than the potential VSS in the period Ta, so that thesource and the drain of the transistor 103 can interchange with eachother. As a result, deterioration of the transistor 103 can besuppressed.

The sequential circuit in FIG. 3 can have advantageous effects similarto those of the above basic circuit.

In the case where a load is connected to the wiring 12, the load isdriven by the transistors 101 and 103. Therefore, W/L of the transistor101 is preferably higher than that of the transistors 102, 104, and 105.Moreover, W/L of the transistor 103 is preferably higher than that ofthe transistors 102, 104, and 105.

Both the transistor 101 and the transistor 103 supply charge to thewiring 12. Note that Vgs of the transistor 101 in the period Tb is oftenlower than Vgs of the transistor 103 in the period Tc; therefore, W/L ofthe transistor 101 is preferably higher than that of the transistor 103.

Both the transistor 102 and the transistor 105 supply charge to the nodeN1. Note that Vgs of the transistor 105 in the period Tc decreasesgradually, whereas Vgs of the transistor 102 in the period Ta is kepthigh. Therefore, W/L of the transistor 105 is preferably higher thanthat of the transistor 102.

Note that W/L of the transistor 102 is preferably higher than that ofthe transistor 104.

Note that the high-level potential of the signal RE may be higher thanthe potential VDD. In this case, the transistor 105 can be preventedfrom being turned off in the period Tc, so that the potential of thenode N1 can increase to the potential VDD.

The low-level potential of the signal RE may be lower than the potentialVSS, in which case the transistor 105 can be definitely turned off evenif the transistor 105 is a normally-on transistor.

The wiring 15 may be supplied with a potential lower than the potentialVDD.

The transistor 103 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the wiring 12.

The transistor 104 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the node N2.

The transistor 105 may be replaced with a switching element having afirst terminal connected to the wiring 15 and a second terminalconnected to the node N1.

Next, a shift register circuit including the sequential circuit in FIG.3 will be described.

First, the configuration of a shift register circuit in this embodimentwill be described with reference to FIG. 5. The shift register circuitin FIG. 5 includes N sequential circuits 100 (N: a natural number). Forconvenience, FIG. 5 only shows first to third sequential circuits 100(indicated by 100[1] to 100[3]) among the N sequential circuits 100.

The shift register circuit is connected to N wirings 21 (indicated by21[1] to 21[N]), a wiring 22, a wiring 23, and a wiring 24.Specifically, in the i-th sequential circuit 100 (i: any one of 2 toN−1), the first terminal of the transistor 101 is connected to thewiring 22 or the wiring 23; the second terminal of the transistor 101 isconnected to the wiring 21[i]; the gate of the transistor 102 isconnected to the wiring 21[i−1]; and the gate of the transistor 105 isconnected to the wiring 21[i+1].

The connection in the first sequential circuit 100 is similar to that inthe i-th sequential circuit 100 except that there is no counterpart ofthe wiring 21 to which the gate of the transistor 102 is connectedbecause there is no preceding sequential circuit 100. Hence, in thefirst sequential circuit 100, the gate of the transistor 102 isconnected to the wiring 24.

The connection in the N-th sequential circuit 100 is similar to that inthe i-th sequential circuit 100 except that there is no counterpart ofthe wiring 21 to which the gate of the transistor 105 is connectedbecause there is no following sequential circuit 100. Hence, in the N-thsequential circuit 100, the gate of the transistor 105 is connected tothe wiring 24. Alternatively, in the N-th sequential circuit 100, thegate of the transistor 105 may be connected to, for example, a wiringsupplied with a reset pulse or an output of a dummy circuit providedfollowing the N-th sequential circuit 100.

In the case where the first terminal of the transistor 101 in theodd-numbered sequential circuit 100 is connected to one of the wirings22 and 23, the first terminal of the transistor 101 in the even-numberedsequential circuit 100 is connected to the other of the wirings 22 and23.

The wiring 21 (also referred to as signal line) outputs a signal SOUTand has a function of transmitting or supplying the signal SOUT. In thei-th sequential circuit 100, the i-th wiring 21 corresponds to thewiring 12, the (i−1)th wiring 21 corresponds to the wiring 14, and the(i+1)th wiring 21 corresponds to the wiring 16. The signal SOUT outputfrom the i-th wiring 21 corresponds to the signal OUT, the signal SOUToutput from the (i−1)th wiring 21 corresponds to the signal SP, and thesignal SOUT output from the (i+1)th wiring 21 corresponds to the signalRE.

The wiring 22 (also referred to as signal line) is supplied with asignal SCK and has a function of transmitting or supplying the signalSCK. In one of the odd-numbered sequential circuit 100 and theeven-numbered sequential circuit 100, the wiring 22 corresponds to thewiring 11 and the signal SCK corresponds to the signal CK.

The wiring 23 (also referred to as signal line) is supplied with asignal SCKB and has a function of transmitting or supplying the signalSCKB. In the other of the odd-numbered sequential circuit 100 and theeven-numbered sequential circuit 100, the wiring 23 corresponds to thewiring 11 and the signal SCKB corresponds to the signal CK. Note thatthe signal SCKB is an inverted signal of the signal SCK or a signal thatis out of phase with the signal SCK.

The wiring 24 (also referred to as signal line) is supplied with asignal SSP and has a function of transmitting or supplying the signalSSP. In the first sequential circuit 100, the wiring 24 corresponds tothe wiring 14 and the signal SSP corresponds to the signal SP.

The shift register circuit in FIG. 5 can have advantageous effectssimilar to those of the above-described basic circuit or sequentialcircuit.

Note that in the i-th sequential circuit 100, the gate of the transistor102 may be connected to the (i−2)th wiring 21 or the (i−3)th wiring 21.

In the i-th sequential circuit 100, the gate of the transistor 105 maybe connected to the (i+2)th wiring 21 or the (i+3)th wiring 21.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 2

In this embodiment, a basic circuit including a buffer circuit and asequential circuit including the basic circuit will be described.

First, the configuration of a basic circuit in this embodiment will bedescribed with reference to FIG. 6A. The basic circuit in FIG. 6A has aconfiguration in which a transistor 201 is provided in the basic circuitin FIG. 1A.

Note that the transistor 201 preferably has the same conductivity typeas the transistor 101. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

A first terminal of the transistor 201 is connected to the wiring 11. Asecond terminal of the transistor 201 is connected to a wiring 31. Agate of the transistor 201 is connected to the gate of the transistor101.

The transistor 201 has a function of controlling electrical continuitybetween the wiring 11 and the wiring 31, a function of supplying thesignal CK of the wiring 11 to the wiring 31, and a function ofmaintaining a potential difference between the wiring 31 and the nodeN2.

The wiring 31 (also referred to as signal line) outputs a signal BOUTand has a function of transmitting or supplying the signal BOUT. Thesignal BOUT is a signal having high and low levels. Moreover, the signalBOUT is an output signal of the basic circuit in FIG. 6A. The signalBOUT corresponds to one of a plurality of output signals output from ashift register circuit or an output signal of a sequential circuit.

Next, a method for driving the basic circuit in FIG. 6A will bedescribed.

Note that the description common to the method for driving the basiccircuit in FIG. 1A is omitted.

The following description is made assuming that an initial potential ofthe wiring 31 is the potential VSS.

For convenience, the assumption is made that when the potential of thenode N2 becomes a potential with which the transistor 101 is turned on,the transistor 201 is also turned on.

First, in the period Ta, the potential of the node N2 exceeds apotential obtained by adding the potential (e.g., the potential VSS) ofthe wiring 11 and the threshold voltage of the transistor 101. Thus, thetransistor 201 is turned on, so that the signal CK of the wiring 11 issupplied to the wiring 31. Since the signal CK is low, the potential ofthe wiring 31 is kept at the potential VSS. That is, the signal BOUTbecomes low.

Then, in the period Tb, the potential of the node N2 exceeds a potentialobtained by adding the potential (e.g., the potential VDD) of the wiring11 and the threshold voltage of the transistor 101. Thus, the transistor201 is kept on, so that the signal CK of the wiring 11 continues to besupplied to the wiring 31. Since the signal CK is high, the potential ofthe wiring 31 increases to the potential VDD. That is, the signal BOUTbecomes high.

In the basic circuit in FIG. 6A, the potential of the wiring 31 can beprevented from decreasing from the potential VSS in a period duringwhich the signal SP is high. Accordingly, the basic circuit in FIG. 6Acan output a more stable signal.

The basic circuit in FIG. 6A can have advantageous effects similar tothose of the basic circuit in Embodiment 1.

In the case where a load is connected to the wiring 31, the load isdriven by the transistor 201. The load connected to the wiring 31 isoften larger than a load connected to the wiring 12. Therefore, W/L ofthe transistor 201 is preferably higher than that of the transistor 101.

The first terminal of the transistor 101 and the first terminal of thetransistor 201 may be connected to different wirings.

Next, a sequential circuit including the basic circuit in FIG. 6A willbe described.

First, the configuration of a sequential circuit in this embodiment willbe described with reference to FIG. 6B. The sequential circuit in FIG.6B has a configuration in which the transistor 201 and a transistor 202are provided in the sequential circuit in FIG. 3.

Note that the transistors 201 and 202 preferably have the sameconductivity type as the transistor 101. In this embodiment, the casewhere these transistors are n-channel transistors will be described.

The first terminal of the transistor 201 is connected to the wiring 11.The second terminal of the transistor 201 is connected to the wiring 31.The gate of the transistor 201 is connected to the gate of thetransistor 101.

A first terminal of the transistor 202 is connected to the wiring 13. Asecond terminal of the transistor 202 is connected to the wiring 31. Agate of the transistor 202 is connected to the node N1.

The transistor 202 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 31, and a function of supplying thepotential VSS of the wiring 13 to the wiring 31.

Next, a method for driving the sequential circuit in FIG. 6B will bedescribed.

Note that the description common to the method for driving thesequential circuit in FIG. 3 is omitted.

The following description is made assuming that an initial potential ofthe wiring 31 is the potential VSS.

For convenience, the assumption is made that when the potential of thenode N2 becomes a potential with which the transistor 101 is turned on,the transistor 201 is also turned on.

Further, for convenience, the assumption is made that when the potentialof the node N1 becomes a potential with which the transistor 103 isturned on, the transistor 202 is also turned on.

First, in the period Ta, the potential of the node N1 becomes thepotential VSS, so that the transistor 202 is turned off. Moreover, thepotential of the node N2 exceeds a potential obtained by adding thepotential (e.g., the potential VSS) of the wiring 11 and the thresholdvoltage of the transistor 101. Thus, the transistor 201 is turned on, sothat the signal CK of the wiring 11 is supplied to the wiring 31. Sincethe signal CK is low, the potential of the wiring 31 is kept at thepotential VSS. That is, the signal BOUT becomes low.

Then, in the period Tb, the potential of the node N1 is kept at thepotential in the period Ta, whereby the transistor 202 is kept off.Further, the potential of the node N2 exceeds a potential obtained byadding the potential (e.g., the potential VDD) of the wiring 11 and thethreshold voltage of the transistor 101. Thus, the transistor 201 isturned on, so that the signal CK of the wiring 11 continues to besupplied to the wiring 31. Since the signal CK is high, the potential ofthe wiring 31 increases to the potential VDD. That is, the signal BOUTbecomes high.

Then, in the period Tc, the potential of the node N2 becomes thepotential VSS, so that the transistor 201 is turned off. Moreover, thepotential of the node N1 increases to a potential obtained bysubtracting the threshold voltage of the transistor 105 from the gatepotential (e.g., the potential VDD) of the transistor 105. Thus, thetransistor 202 is turned on, whereby the potential of the wiring 13 issupplied to the wiring 31. Consequently, the potential of the wiring 31decreases to the potential VSS. That is, the signal BOUT becomes low.

Then, in the period Td, the potential of the node N2 is kept at thepotential VSS, so that the transistor 201 remains off. Moreover, thepotential of the node N1 is kept at the potential in the period Tc.Thus, the transistor 202 is kept on, whereby the potential of the wiring13 continues to be supplied to the wiring 31. Consequently, thepotential of the wiring 31 is kept at the potential VSS. That is, thesignal BOUT remains low.

The sequential circuit in FIG. 6B can have advantageous effects similarto those of the above basic circuit, and the basic circuit and thesequential circuit in Embodiment 1.

In the case where a load is connected to the wiring 31, the load isdriven by the transistor 201. The load connected to the wiring 31 isoften larger than a load connected to the wiring 12. Therefore, W/L ofthe transistor 202 is preferably higher than that of the transistor 103.

Both the transistor 201 and the transistor 202 supply charge to thewiring 31. Note that Vgs of the transistor 201 in the period Tb is oftenlower than Vgs of the transistor 202 in the period Tc; therefore, W/L ofthe transistor 201 is preferably higher than that of the transistor 202.

The transistor 202 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the wiring 31.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 3

In this embodiment, sequential circuits different from those inEmbodiments 1 and 2 will be described.

A sequential circuit in FIG. 7A has a configuration in which the secondterminal of the transistor 105 is connected to the wiring 16 in thesequential circuit in FIG. 3.

In the sequential circuit in FIG. 7A, the wiring 15 and the potentialVDD can be omitted.

Note that in any of the sequential circuits and the shift registercircuit described in Embodiments 1 and 2, the second terminal of thetransistor 105 may be connected to the wiring 16.

A sequential circuit in FIG. 7B has a configuration in which the secondterminal of the transistor 105 in the sequential circuit in FIG. 3 isconnected to a wiring 17.

The wiring 17 (also referred to as signal line) is supplied with asignal CKB and has a function of transmitting or supplying the signalCKB. The signal CKB is a signal having high and low levels. The signalCKB corresponds to one of a plurality of clock signals input to a shiftregister circuit. Moreover, the signal CKB is an inverted signal of thesignal CK or a signal that is out of phase with the signal CK.

In the sequential circuit in FIG. 7B, the wiring 15 and the potentialVDD can be omitted.

Note that in any of the sequential circuits and the shift registercircuit described in Embodiments 1 and 2, the second terminal of thetransistor 105 may be connected to the wiring 17.

A sequential circuit in FIG. 8A has a configuration in which a capacitor301 is provided in the sequential circuit in FIG. 3.

A first electrode of the capacitor 301 is connected to the wiring 13. Asecond electrode of the capacitor 301 is connected to the node N1.

The capacitor 301 has a function of maintaining a potential differencebetween the wiring 13 and the node N1, and a function of maintaining thepotential of the node N1.

In the period Ta, the capacitor 301 maintains a potential differencebetween the wiring 13 and the node N1 at the time when the potential VSSof the wiring 13 is supplied to the node N1.

In the period Tb, the capacitor 301 holds the voltage in the period Ta.

In the period Tc, the capacitor 301 maintains a potential differencebetween the wiring 13 and the node N1 at the time when the potential VDDof the wiring 15 is supplied to the node N1.

In the period Td, the capacitor 301 holds the voltage in the period Tc.

In the sequential circuit in FIG. 8A, the rise in the potential of thenode N1 associated with the rise in the potential of the wiring 12 canbe suppressed because the capacitor 301 maintains a potential differencebetween the wiring 13 and the node N1 in the period Tb.

Furthermore, the change in the potential of the node N1 can besuppressed because the capacitor 301 maintains a potential differencebetween the wiring 13 and the node N1 in the period Td.

Note that the first electrode of the capacitor 301 is not limited tobeing connected to the wiring 13 and may be connected to the wiring 11,the wiring 14, the wiring 15, or the wiring 16, for example.

Note that the capacitor 301 may be provided in any of the basiccircuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

A sequential circuit in FIG. 8B has a configuration in which atransistor 302 is provided in the sequential circuit in FIG. 3.

Note that the transistor 302 preferably has the same conductivity typeas the transistor 101. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

A first terminal of the transistor 302 is connected to the wiring 13. Asecond terminal of the transistor 302 is connected to the node N2. Agate of the transistor 302 is connected to the wiring 16.

The transistor 302 has a function of controlling electrical continuitybetween the wiring 13 and the node N2, and a function of supplying thepotential VSS of the wiring 13 to the node N2.

In the period Ta, the period Tb, and the period Td, the signal RE islow; thus, the transistor 302 is off.

In the period Tc, the signal RE goes high. When the signal RE goes high,the transistor 302 is turned on, and the potential VSS of the wiring 13is supplied to the node N2.

Since the sequential circuit in FIG. 8B includes the transistor 302, thetiming at which the potential VSS of the wiring 13 starts to be suppliedto the node N2 can be advanced in the period Tc. Thus, the timing atwhich the potential of the node N2 starts to fall can be advanced, sothat the timing at which the transistor 101 is turned off can beadvanced.

Note that the transistor 302 may be provided in any of the basiccircuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

The transistor 302 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the node N2.

A sequential circuit in FIG. 9A has a configuration in which atransistor 303 is provided in the sequential circuit in FIG. 3.

A first terminal of the transistor 303 is connected to the wiring 13. Asecond terminal of the transistor 303 is connected to the wiring 12. Agate of the transistor 303 is connected to the wiring 16.

Note that the transistor 303 preferably has the same conductivity typeas the transistor 101. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

The transistor 303 has a function of controlling electrical continuitybetween the wiring 13 and the wiring 12, and a function of supplying thepotential VSS of the wiring 13 to the wiring 12.

In the period Ta, the period Tb, and the period Td, the signal RE islow; thus, the transistor 303 is off.

In the period Tc, the signal RE goes high. When the signal RE goes high,the transistor 303 is turned on, and the potential VSS of the wiring 13is supplied to the wiring 12.

Since the sequential circuit in FIG. 9A includes the transistor 303, thetiming at which the potential VSS of the wiring 13 starts to be suppliedto the wiring 12 can be advanced in the period Tc. As a result, the falltime of the signal OUT can be shortened.

Note that the transistor 303 may be provided in any of the basiccircuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

In the case where the transistor 303 is provided particularly in thebasic circuit and the sequential circuit described in Embodiment 2, thesecond terminal of the transistor 303 may be connected to the wiring 31.Alternatively, in addition to the transistor 303, a transistor may beprovided so that its first terminal is connected to the wiring 13, itssecond terminal is connected to the wiring 31, and its gate is connectedto the wiring 16. As a result, the fall time of the signal BOUT can beshortened.

The transistor 303 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the wiring 12 or the wiring 31.

A sequential circuit in FIG. 9B has a configuration in which atransistor 304 is provided in the sequential circuit in FIG. 3.

Note that the transistor 304 preferably has the same conductivity typeas the transistor 101. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

A first terminal of the transistor 304 is connected to the wiring 15. Asecond terminal of the transistor 304 is connected to the node N2. Agate of the transistor 304 is connected to the wiring 14.

The transistor 304 has a function of controlling electrical continuitybetween the wiring 15 and the node N2, a function of supplying thepotential VDD of the wiring 15 to the node N2, and a function of raisingthe potential of the node N2 and then stopping supply of charge, apotential, a signal, or the like to the node N2.

In the period Ta, the signal SP goes high. When the signal SP goes high,the transistor 304 is turned on. When the transistor 304 is turned on,the potential VDD of the wiring 15 is supplied to the node N2. Thus, thepotential of the node N2 rises. When the potential of the node N2becomes a potential obtained by subtracting the threshold voltage of thetransistor 304 from the gate potential (e.g., the potential VDD) of thetransistor 304, the transistor 304 is turned off. When the transistor304 is turned off, the node N2 becomes floating.

In the periods Tb, Tc, and Td, the signal SP is low. The transistor 304is turned off when the signal SP goes low.

The sequential circuit in FIG. 9B includes the transistor 304, wherebythe potential of the node N2 can be definitely increased in the periodTa. That is, the transistor 101 can be definitely turned on.

Note that the transistor 304 may be provided in any of the basiccircuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

Note that the first terminal of the transistor 304 may be connected tothe wiring 11, the wiring 14, or the wiring 17, for example.

The transistor 304 may be replaced with a switching element having afirst terminal connected to the wiring 15 or the wiring 11 and a secondterminal connected to the node N2.

A sequential circuit in FIG. 10A has a configuration in which atransistor 305 is provided in the sequential circuit in FIG. 3.

Note that the transistor 305 preferably has the same conductivity typeas the transistor 101. In this embodiment, the case where thesetransistors are n-channel transistors will be described.

A first terminal of the transistor 305 is connected to the wiring 16. Asecond terminal of the transistor 305 is connected to the gate of thetransistor 105. A gate of the transistor 305 is connected to the wiring15.

The transistor 305 has a function of controlling electrical continuitybetween the wiring 16 and the gate of the transistor 105, a function ofsupplying the signal RE of the wiring 16 to the gate of the transistor105, and a function of raising the gate potential of the transistor 105and then stopping supply of charge, a signal, a potential, or the liketo the gate of the transistor 105.

In the period Ta, the period Tb, and the period Td, the transistor 305is on. When the transistor 305 is turned on, the signal RE of the wiring16 is supplied to the gate of the transistor 105. Since the signal RE islow, the gate potential of the transistor 305 becomes the potential VSS.

In the period Tc, the transistor 305 is turned on. When the transistor305 is turned on, the signal RE of the wiring 16 is supplied to the gateof the transistor 105. Since the signal RE is high, the gate potentialof the transistor 105 rises. When the gate potential of the transistor105 rises, the transistor 105 is turned on. When the transistor 105 isturned on, the potential VDD of the wiring 15 is supplied to the node N1and the potential of the node N1 rises as a result. When the gatepotential of the transistor 105 becomes a potential obtained bysubtracting the threshold voltage of the transistor 305 from the gatepotential (e.g., the potential VDD) of the transistor 305, thetransistor 305 is turned off. Thus, the gate of the transistor 105becomes floating. At this time, a potential difference between the gateof the transistor 105 and the node N1 is maintained between the gate andthe second terminal of the transistor 105. Accordingly, the gatepotential of the transistor 105 rises as the potential of the node N1rises. The transistor 105 is kept on when the gate potential of thetransistor 105 exceeds a potential obtained by adding the potential VDDof the wiring 15 and the threshold voltage of the transistor 105. Thus,the potential of the node N1 becomes the potential VDD.

The sequential circuit in FIG. 10A includes the transistor 305, wherebythe potential of the node N1 can increase to the potential VDD in theperiod Tc. Thus, Vgs of the transistors 103 and 104 can be increased,which makes it possible to turn on the transistors 103 and 104 moredefinitely.

Note that the transistor 305 may be provided in any of the basiccircuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

The transistor 305 may be replaced with a switching element having afirst terminal connected to the wiring 16 and a second terminalconnected to the gate of the transistor 105.

Note that it is possible to provide a capacitor having a first electrodeconnected to the gate of the transistor 105 and a second electrodeconnected to the second terminal of the transistor 105. Accordingly, thecapacitance between the gate and the second terminal of the transistor105 can be increased, so that the gate potential of the transistor 105can be further increased.

Note also that the gate of the transistor 305 may be connected to thewiring 17.

A sequential circuit in FIG. 10B has a configuration in which atransistor 306 and a transistor 307 are provided in the sequentialcircuit in FIG. 3.

Note that the transistors 306 and 307 preferably have the sameconductivity type as the transistor 101. In this embodiment, the casewhere these transistors are n-channel transistors will be described.

A first terminal of the transistor 306 is connected to the wiring 15. Asecond terminal of the transistor 306 is connected to the gate of thetransistor 105. A gate of the transistor 306 is connected to the wiring16.

A first terminal of the transistor 307 is connected to the wiring 13. Asecond terminal of the transistor 307 is connected to the gate of thetransistor 105. A gate of the transistor 307 is connected to the wiring14.

The transistor 306 has a function of controlling electrical continuitybetween the wiring 15 and the gate of the transistor 105, a function ofsupplying the potential VDD of the wiring 15 to the gate of thetransistor 105, and a function of raising the gate potential of thetransistor 105 and then stopping supply of charge, a potential, asignal, or the like to the gate of the transistor 105.

The transistor 307 has a function of controlling electrical continuitybetween the wiring 13 and the gate of the transistor 105, and a functionof supplying the potential VSS of the wiring 13 to the gate of thetransistor 105.

In the period Ta, the signal SP goes high and the signal RE goes low.When the signal RE goes low, the transistor 306 is turned off. When thesignal SP goes high, the transistor 307 is turned on. When thetransistor 307 is turned on, the potential VSS of the wiring 13 issupplied to the gate of the transistor 105. Thus, the gate potential ofthe transistor 105 becomes the potential VSS, and the transistor 105 isturned off.

In the period Tb, the signal SP goes low and the signal RE is kept low.When the signal SP goes low, the transistor 306 is turned off. Thetransistor 307 remains off while the signal RE is kept low. When both ofthe transistors 306 and 307 are turned off in such a manner, the gate ofthe transistor 105 becomes floating. Thus, the gate potential of thetransistor 105 is kept at the potential VSS, so that the transistor 105remains off.

In the period Tc, the signal SP is kept low and the signal RE goes high.The transistor 307 remains off while the signal SP is kept low. When thesignal RE goes high, the transistor 306 is turned on. When thetransistor 306 is turned on, the potential VDD of the wiring 15 issupplied to the gate of the transistor 105, and the gate potential ofthe transistor 105 rises. When the gate potential of the transistor 105rises, the transistor 105 is turned on. When the transistor 105 isturned on, the potential VDD of the wiring 15 is supplied to the node N1and the potential of the node N1 rises as a result. When the gatepotential of the transistor 105 becomes a potential obtained bysubtracting the threshold voltage of the transistor 306 from the gatepotential (e.g., the potential VDD) of the transistor 306, thetransistor 306 is turned off. Thus, the gate of the transistor 105becomes floating. At this time, a potential difference between the gateof the transistor 105 and the node N1 is maintained between the gate andthe second terminal of the transistor 105. Accordingly, the gatepotential of the transistor 105 rises as the potential of the node N1rises. The transistor 105 remains on when the gate potential of thetransistor 105 exceeds a potential obtained by adding the potential VDDof the wiring 15 and the threshold voltage of the transistor 105. Thus,the potential of the node N1 becomes the potential VDD.

In the period Td, the signal SP is kept low and the signal RE goes low.The transistor 306 remains off while the signal SP is kept low. When thesignal RE goes low, the transistor 307 is turned off. When both of thetransistors 306 and 307 are turned off in such a manner, the gate of thetransistor 105 becomes floating. Thus, the gate potential of thetransistor 105 is kept at the potential in the period Tc, so that thetransistor 105 remains on.

The sequential circuit in FIG. 10B includes the transistors 306 and 307,whereby the potential of the node N1 can increase to the potential VDDin the period Tc. Thus, Vgs of the transistors 103 and 104 can beincreased, which makes it possible to turn on the transistors 103 and104 more definitely.

In the period Td, the transistor 105 can be kept on, so that thepotential VDD of the wiring 15 can continue to be supplied to the nodeN1. Consequently, the potential of the node N1 can be stably maintained.

Note that the transistors 306 and 307 may be provided in any of thebasic circuits, the sequential circuits, and the shift register circuitdescribed in Embodiments 1 to 3.

The transistor 306 may be replaced with a switching element having afirst terminal connected to the wiring 15 and a second terminalconnected to the gate of the transistor 105.

The transistor 307 may be replaced with a switching element having afirst terminal connected to the wiring 13 and a second terminalconnected to the gate of the transistor 105.

Note that the first terminal of the transistor 306 may be connected tothe wiring 16 or the wiring 17, for example.

A sequential circuit in FIG. 11A has a configuration in which a circuit308 is provided in the sequential circuit in FIG. 3.

A first terminal of the circuit 308 is connected to the node N1. Asecond terminal of the circuit 308 is connected to the gate of thetransistor 104.

The circuit 308 has a function of outputting a distorted potential orsignal of the first terminal from the second terminal; a function ofoutputting, from the second terminal, a potential or signal of the firstterminal with a prolonged rise time and/or fall time; and a function ofoutputting a delayed potential or signal of the first terminal from thesecond terminal.

In the period Ta, the potential of the wiring 12 decreases as thepotential of the node N1 decreases. At this time, the transistor 104 ison because the gate potential of the transistor 104 starts to decreaseafter the decrease in the potential of the node N1 starts or because thegate potential of the transistor 104 decreases more slowly than thepotential of the node N1. Thus, the potential VSS of the wiring 13 issupplied to the gate of the transistor 101. Then, the gate potential ofthe transistor 104 falls, and the transistor 104 is turned off.

In the period Tc, the potential of the node N1 rises. At this time, thetransistor 104 is off because the gate potential of the transistor 104starts to rise after the rise in the potential of the node N1 starts orbecause the gate potential of the transistor 104 rises more slowly thanthe potential of the node N1. Then, the gate potential of the transistor104 rises, and the transistor 104 is turned on. Thus, the potential VSSof the wiring 13 is supplied to the gate of the transistor 101, and thetransistor 101 is turned off.

In the period Tb and the period Td, the operation of the sequentialcircuit in FIG. 11A is similar to that of the sequential circuit in FIG.3 because the potential of the node N1 does not rise or fallsignificantly.

In the sequential circuit in FIG. 11A, the potential VSS of the wiring13 can be supplied to the node N2 while the potential of the wiring 12decreases in the period Ta. Consequently, the decrease in the potentialof the node N2 associated with the decrease in the potential of thewiring 12 can be prevented. Preventing the decrease in the potential ofthe node N2 can further increase the potential of the node N2, wherebyVgs of the transistor 101 can be further increased.

Further, the timing at which the transistor 101 is turned off can bedelayed in the period Tc, so that the signal CK of the wiring 11 can besupplied to the wiring 12. Since the signal CK is low, the fall time ofthe signal OUT can be shortened. The fall time of the signal OUT can bedrastically shortened because W/L of the transistor 101 is particularlyhigh in many cases.

Here, specific examples of the circuit 308 will be described.

The circuit 308 in FIG. 11B has a transistor 308 a. A first terminal ofthe transistor 308 a is connected to the first terminal of the circuit308. A second terminal of the transistor 308 a is connected to thesecond terminal of the circuit 308. A gate of the transistor 308 a isconnected to the wiring 11.

The circuit 308 in FIG. 11C has a configuration in which a transistor308 b is provided in the circuit 308 in FIG. 11B. A first terminal ofthe transistor 308 b is connected to the first terminal of the circuit308. A second terminal of the transistor 308 b is connected to thesecond terminal of the circuit 308. A gate of the transistor 308 b isconnected to the second terminal of the circuit 308.

The circuit 308 in FIG. 11D has a configuration in which a transistor308 c is provided in the circuit 308 in FIG. 11B. A first terminal ofthe transistor 308 c is connected to the wiring 11. A second terminal ofthe transistor 308 c is connected to the second terminal of the circuit308. A gate of the transistor 308 c is connected to the first terminalof the circuit 308.

The circuit 308 in FIG. 11E has a configuration in which a transistor308 d and a transistor 308 e are provided in the circuit 308 in FIG.11B. A first terminal of the transistor 308 d is connected to the wiring11. A second terminal of the transistor 308 d is connected to the secondterminal of the circuit 308. A first terminal of the transistor 308 e isconnected to the first terminal of the circuit 308. A second terminal ofthe transistor 308 e is connected to a gate of the transistor 308 d. Agate of the transistor 308 e is connected to the wiring 11.

In the circuit 308 in FIG. 11E, the gate potential of the transistor 308d can be higher than the potential VDD, so that the potential of thesecond terminal of the circuit 308 can increase to the potential VDD.

Note that the transistors 308 a to 308 e preferably have the sameconductivity type as the transistor 101.

The gate of the transistor 308 a, the first terminal of the transistor308 c, the first terminal of the transistor 308 d, and/or the gate ofthe transistor 308 e may be connected to the wiring 17, for example.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 4

Using an EL display device as an example, cross-sectional structures ofa pixel and a driver circuit of a display device according to oneembodiment of the present invention will be described with reference toFIG. 12. FIG. 12 exemplifies cross-sectional structures of a pixel 840and a driver circuit 841.

The pixel 840 includes a light-emitting element 832 and a transistor 831having a function of supplying current to the light-emitting element832. In addition to the light-emitting element 832 and the transistor831, the pixel 840 may also include a variety of semiconductor elementssuch as a transistor that controls input of an image signal to the pixel840 and a capacitor that holds the potential of an image signal.

The driver circuit 841 includes a transistor 830 and a capacitor 833 forholding the gate voltage of the transistor 830. The driver circuit 841corresponds to any of the basic circuits, the sequential circuits, andthe shift register circuit described in Embodiments 1 to 3.Specifically, the transistor 830 corresponds to the transistor 101 orthe transistor 201, for example. The driver circuit 841 may also includea variety of semiconductor elements such as a transistor and a capacitorin addition to the transistor 830 and the capacitor 833.

The transistor 831 includes, over a substrate 800 having an insulatingsurface, a conductive film 816 functioning as a gate, a gate insulatingfilm 802 over the conductive film 816, a semiconductor film 817 thatoverlaps the conductive film 816 with the gate insulating film 802placed therebetween, and conductive films 815 and 818 that arepositioned over the semiconductor film 817 and function as a sourceterminal and a drain terminal. The conductive film 816 also functions asa scan line.

The transistor 830 includes, over the substrate 800 having an insulatingsurface, a conductive film 812 functioning as a gate, the gateinsulating film 802 over the conductive film 812, a semiconductor film813 that overlaps the conductive film 812 with the gate insulating film802 placed therebetween, and conductive films 814 and 819 that arepositioned over the semiconductor film 813 and function as a sourceterminal and a drain terminal.

The capacitor 833 includes, over the substrate 800 having an insulatingsurface, the conductive film 812, the gate insulating film 802 over theconductive film 812, and the conductive film 819 that overlaps theconductive film 812 with the gate insulating film 802 placedtherebetween.

An insulating film 820 and an insulating film 821 are stacked in thisorder over the conductive films 814, 815, 818, and 819. A conductivefilm 822 functioning as an anode is formed over the insulating film 821.The conductive film 822 is connected to the conductive film 818 througha contact hole 823 formed in the insulating films 820 and 821.

An insulating film 824 having an opening where part of the conductivefilm 822 is exposed is provided over the insulating film 821. An ELlayer 825 and a conductive film 826 functioning as a cathode are stackedin this order over the part of the conductive film 822 and theinsulating film 824. A region where the conductive film 822, the ELlayer 825, and the conductive film 826 overlap one another correspondsto the light-emitting element 832.

In one embodiment of the present invention, the transistors 830 and 831may include a semiconductor film containing an amorphous,microcrystalline, polycrystalline, or single crystal semiconductor(e.g., silicon or germanium), or a semiconductor film containing a widebandgap semiconductor such as an oxide semiconductor.

When the semiconductor films of the transistors 830 and 831 are formedusing an amorphous, microcrystalline, polycrystalline, or single crystalsemiconductor (e.g., silicon or germanium), impurity regions functioningas source and drain terminals are formed by addition of an impurityelement imparting one conductivity to the semiconductor films. Forexample, an impurity region having n-type conductivity can be formed byaddition of phosphorus or arsenic to the semiconductor film. Further, animpurity region having p-type conductivity can be formed by addition ofboron, for example, to the semiconductor film.

In the case where an oxide semiconductor is used for the semiconductorfilms of the transistors 830 and 831, impurity regions functioning assource and drain terminals may be formed by addition of a dopant to thesemiconductor films. The dopant can be added by ion implantation.Examples of the dopant are a rare gas such as helium, argon, and xenon;and a Group 15 element such as nitrogen, phosphorus, arsenic, andantimony. For example, when nitrogen is used as the dopant, theconcentration of nitrogen atoms in the impurity region preferably rangesfrom 5×10¹⁹/cm³ to 1×10²²/cm³.

As a silicon semiconductor, any of the following can be used, forexample: amorphous silicon formed by sputtering or vapor phase growthsuch as plasma CVD, polycrystalline silicon obtained in such a mannerthat amorphous silicon is crystallized by laser annealing or the like,and single crystal silicon obtained in such a manner that a surfaceportion of a single crystal silicon wafer is separated afterimplantation of hydrogen ions or the like into the silicon wafer.

The oxide semiconductor film contains at least one element selected fromIn, Ga, Sn, and Zn. Examples of the oxide semiconductor are an oxide offour metal elements, such as an In—Sn—Ga—Zn—O-based oxide semiconductor;oxides of three metal elements, such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and aSn—Al—Zn—O-based oxide semiconductor; oxides of two metal elements, suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, and an In—Ga—O-based material; andoxides of one metal element, such as an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor.In addition, any of the above oxide semiconductors may contain anelement other than In, Ga, Sn, and Zn, for example, SiO₂.

For example, an In—Ga—Zn—O-based oxide semiconductor refers to an oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn), andthere is no limitation on the composition thereof.

For the oxide semiconductor film, a thin film expressed by a chemicalformula of InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one ormore metal elements selected from Zn, Ga, Al, Mn, and Co. For example, Mcan be Ga, Ga and Al, Ga and Mn, or Ga and Co.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, the atomic ratio of metal elements in a target to be usedis In:Zn=50:1 to 1:2 (In ₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio),further preferably In:Zn=15:1 to 1.5:1 (In₂O₃:ZnO=15:2 to 3:4 in a molarratio). For example, in a target used for forming an In—Zn—O-based oxidesemiconductor with an atomic ratio of In:Zn:O=X:Y:Z, the relation ofZ>1.5X+Y is satisfied. The mobility can be increased by keeping theratio of Zn within the above range.

Note that a purified oxide semiconductor obtained by reduction ofimpurities serving as electron donors (donors), such as moisture orhydrogen, and by reduction of oxygen defects is an i-type (intrinsic)semiconductor or a substantially i-type semiconductor. A transistorincluding the purified oxide semiconductor therefore has extremely lowoff-state current. The band gap of the oxide semiconductor is 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more. Withthe use of an oxide semiconductor film that is highly purified bysufficient decrease in the concentration of impurities such as moistureand hydrogen and reduction of oxygen defects, the off-state current of atransistor can be decreased.

Specifically, various experiments can prove low off-state current of atransistor in which a purified oxide semiconductor is used for asemiconductor film. For example, the off-state current of even atransistor with a channel width of 1×10⁶ μm and a channel length of 10μm can be less than or equal to the measurement limit of a semiconductorparameter analyzer, that is, less than or equal to 1×10⁻¹³ A when thevoltage between a source terminal and a drain terminal (drain voltage)ranges from 1 V to 10 V. In this case, the off-state current densitycorresponding to a value obtained by dividing the off-state current bythe channel width of the transistor is 100 zA/μm or less. In addition,the off-state current has been measured using a circuit in which acapacitor and a transistor were connected to each other and chargeflowing into or from the capacitor was controlled by the transistor. Forthe measurement, the transistor in which a channel formation region isformed in a purified oxide semiconductor film has been used, and theoff-state current density of the transistor has been measured from achange in the amount of charge of the capacitor per unit time. As aresult, it has been proven that a lower off-state current density ofseveral tens of yoctoamperes per micrometer (yA/μm) is obtained at avoltage between the source terminal and the drain terminal of thetransistor of 3 V. Consequently, the off-state current of the transistorin which the channel formation region is formed in the purified oxidesemiconductor film is significantly lower than that of a transistorusing crystalline silicon.

Unless otherwise specified, in this specification, the off-state currentof an n-channel transistor is a current that flows between a sourceterminal and a drain terminal when the potential of a gate is lower thanor equal to 0 with the potential of the source terminal as a referencepotential while the potential of the drain terminal is higher than thoseof the source terminal and the gate. Moreover, in this specification,the off-state current of a p-channel transistor is a current that flowsbetween a source terminal and a drain terminal when the potential of agate is higher than or equal to 0 with the potential of the sourceterminal as a reference potential while the potential of the drainterminal is lower than those of the source terminal and the gate.

An oxide semiconductor film can be formed, for example, by sputteringusing a target including indium (In), gallium (Ga), and zinc (Zn). Whenan In—Ga—Zn-based oxide semiconductor film is formed by sputtering, itis preferable to use an In—Ga—Zn-based oxide target having an atomicratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4. When anoxide semiconductor film is formed using an In—Ga—Zn-based oxide targethaving the aforementioned atomic ratio, a polycrystal or ac-axis-aligned crystal (CAAC), which is described below, is readilyformed.

The filling rate of the target including In, Ga, and Zn is 90% or higherand 100% or lower, preferably 95% or higher and lower than 100%. Withthe use of the target with high filling rate, a dense oxidesemiconductor film is formed.

Specifically, the oxide semiconductor film may be formed as follows: thesubstrate is held in a treatment chamber with pressure reduced, asputtering gas from which hydrogen and moisture are removed isintroduced while residual moisture in the treatment chamber is removed,and the above-described target is used. The substrate temperature duringfilm formation may range from 100° C. to 600° C., preferably from 200°C. to 400° C. By forming the oxide semiconductor film while thesubstrate is heated, the concentration of impurities included in theformed oxide semiconductor film can be reduced. In addition, damage bysputtering can be reduced. In order to remove remaining moisture in thetreatment chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the deposition chamber which is evacuated with thecryopump, for example, a hydrogen atom and a compound containing ahydrogen atom, such as water (H₂O) (preferably, a compound containing acarbon atom as well) are removed, whereby the impurity concentration inthe oxide semiconductor film formed in the chamber can be reduced.

Note that the oxide semiconductor film formed by sputtering or the likesometimes contains a large amount of moisture or hydrogen (including ahydroxyl group) as impurities. Moisture and hydrogen easily form a donorlevel and thus serve as impurities in the oxide semiconductor. In oneembodiment of the present invention, in order to reduce impurities suchas moisture or hydrogen in the oxide semiconductor film (in order toperform dehydration or dehydrogenation), the oxide semiconductor film issubjected to heat treatment in a reduced-pressure atmosphere, an inertgas atmosphere of nitrogen, a rare gas, or the like, an oxygen gasatmosphere, or ultra-dry air (the moisture amount is 20 ppm (−55° C. byconversion into a dew point) or less, preferably 1 ppm or less, furtherpreferably 10 ppb or less in the case where measurement is performed bya dew point meter in a cavity ring-down laser spectroscopy (CRDS)method).

By performing heat treatment on the oxide semiconductor film, moistureor hydrogen in the oxide semiconductor film can be eliminated.Specifically, heat treatment may be performed at a temperature higherthan or equal to 250° C. and lower than or equal to 750° C., preferablyhigher than or equal to 400° C. and lower than the strain point of thesubstrate. For example, heat treatment may be performed at 500° C. forabout 3 to 6 minutes. When an RTA method is used for the heat treatment,dehydration or dehydrogenation can be performed in a short time;therefore, treatment can be performed even at a temperature higher thanthe strain point of a glass substrate.

Note that in some cases, the heat treatment makes oxygen released fromthe oxide semiconductor film and an oxygen defect is formed in the oxidesemiconductor film. To prevent an oxygen defect, an insulating filmincluding oxygen is used as an insulating film in contact with the oxidesemiconductor film, such as a gate insulating film, in one embodiment ofthe present invention. Then, heat treatment is performed after formationof the insulating film including oxygen, so that oxygen is supplied fromthe insulating film to the oxide semiconductor film. With the abovestructure, oxygen defects serving as donors can be reduced in the oxidesemiconductor film and the stoichiometric composition of the oxidesemiconductor included in the oxide semiconductor film can be satisfied.It is preferable that the proportion of oxygen in the oxidesemiconductor film is higher than that in the stoichiometriccomposition. As a result, the oxide semiconductor film can be madesubstantially i-type and variations in electrical characteristics oftransistors due to oxygen defects can be reduced; thus, electricalcharacteristics can be improved.

The heat treatment for supplying oxygen to the oxide semiconductor filmis performed in a nitrogen atmosphere, ultra-dry air, or a rare gas(e.g., argon or helium) atmosphere preferably at temperatures rangingfrom 200° C. to 400° C., for example, from 250° C. to 350° C. The watercontent in the gas is preferably 20 ppm or less, more preferably 1 ppmor less, still more preferably 10 ppb or less.

Note that the oxide semiconductor film is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film is preferably a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystalline parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not clear. Further, a grain boundary in theCAAC-OS film is not found with the TEM. Thus, reduction in electronmobility due to the grain boundary is suppressed in the CAAC-OS film.

In each of the crystal parts included in the CAAC-OS film, the c-axis isaligned in a direction parallel to a normal vector of a surface overwhich the CAAC-OS film is deposited or a normal vector of a surface ofthe CAAC-OS film, triangular or hexagonal atomic arrangement which isseen from the direction perpendicular to the a-b plane is formed, andmetal atoms are arranged in a layered manner or metal atoms and oxygenatoms are arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, theterm “perpendicular” includes a range from 85° to 95°, and the term“parallel” includes a range from −5° to 5° unless otherwise specified.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, when crystal growth occurs from a surface side ofthe oxide semiconductor film in the process of forming the CAAC-OS film,the proportion of crystal parts in the vicinity of the surface of theoxide semiconductor film is sometimes higher than that in the vicinityof the surface over which the oxide semiconductor film is deposited.Further, when an impurity is added to the CAAC-OS film, the crystal partin a region to which the impurity is added becomes amorphous in somecases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of the surface overwhich the CAAC-OS film is deposited or a normal vector of the surface ofthe CAAC-OS film, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS film (thecross-sectional shape of the surface over which the CAAC-OS film isdeposited or the cross-sectional shape of the surface of the CAAC-OSfilm). Note that the direction of c-axis of the crystal part is thedirection parallel to a normal vector of the surface over which theCAAC-OS film is deposited or a normal vector of the surface of theCAAC-OS film. The crystal part is formed during deposition or byperforming treatment for crystallization such as heat treatment afterdeposition.

In a transistor including the CAAC-OS film, a change in electricalcharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

The CAAC-OS film is formed by sputtering using a polycrystalline oxidesemiconductor sputtering target, for example. When ions collide with thesputtering target, a crystal region included in the sputtering targetmight be separated from the target along the a-b plane; in other words,sputtered particles having a plane parallel to the a-b plane (flatplate-like sputtered particles or pellet-like sputtered particles) mightflake off from the sputtering target. In this case, the flat plate-likesputtered particles reach a substrate while maintaining their crystalstate, whereby the CAAC-OS film can be formed.

The CAAC-OS film is preferably deposited under the following conditions.

Deformation of the crystal due to impurities can be prevented byreducing the amount of impurities entering the CAAC-OS film during thedeposition, for example, by reducing the concentration of impurities(e.g., hydrogen, water, carbon dioxide, and nitrogen) that exist in thedeposition chamber or by reducing the concentration of impurities in adeposition gas. Specifically, a deposition gas with a dew point of −80°C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is set from 100° C. to 740°C., preferably from 200° C. to 500° C. By increasing the substrateheating temperature during the deposition, when the flat-plate-likesputtered particle reaches the substrate, migration occurs on thesubstrate surface, so that a flat plane of the flat-plate-like sputteredparticle is attached to the substrate.

It is preferable that the proportion of oxygen in the deposition gas beincreased and the electric power be optimized in order to reduce plasmadamage at the deposition. The proportion of oxygen in the deposition gasis 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound targetwill be described below.

A polycrystalline In—Ga—Zn—O compound target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder at a predetermined molarratio, applying pressure to the mixture, and then performing heattreatment on the mixture at temperatures ranging from 1000° C. to 1500°C. Note that X, Y, and Z are each a given positive number. Here, thepredetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing the powder canbe determined as appropriate depending on the desired sputtering target.

Next, examples of a specific structure of a transistor included in thesemiconductor device according to one embodiment of the presentinvention will be described.

A transistor illustrated in FIG. 13A is a bottom-gate transistor with achannel-etched structure.

The transistor illustrated in FIG. 13A includes a gate electrode (gate)1602 formed on an insulating surface, a gate insulating film 1603 overthe gate electrode 1602, a semiconductor film 1604 that overlaps thegate electrode 1602 with the gate insulating film 1603 placedtherebetween, and conductive films 1605 and 1606 formed over thesemiconductor film 1604. An insulating film 1607 formed over thesemiconductor film 1604 and the conductive films 1605 and 1606 may beconsidered as a component of the transistor.

The transistor in FIG. 13A may also include a backgate electrode thatoverlaps the semiconductor film 1604 with the insulating film 1607placed therebetween.

A transistor illustrated in FIG. 13B is a bottom-gate transistor with achannel protective structure.

The transistor illustrated in FIG. 13B includes a gate electrode 1612formed on an insulating surface, a gate insulating film 1613 over thegate electrode 1612, a semiconductor film 1614 that overlaps the gateelectrode 1612 with the gate insulating film 1613 placed therebetween, achannel protective film 1618 formed over the semiconductor film 1614,and conductive films 1615 and 1616 formed over the semiconductor film1614. An insulating film 1617 formed over the channel protective film1618 and the conductive films 1615 and 1616 may be considered as acomponent of the transistor.

The transistor in FIG. 13B may also include a backgate electrode thatoverlaps the semiconductor film 1614 with the insulating film 1617placed therebetween.

The channel protective film 1618 can prevent the portion serving as achannel formation region in the semiconductor film 1614 from beingdamaged in a later step (e.g., from being reduced in thickness by plasmaor an etchant in etching). As a result, the reliability of thetransistor can be improved.

A transistor illustrated in FIG. 13C is a bottom-gate bottom-contacttransistor.

The transistor illustrated in FIG. 13C includes a gate electrode 1622formed on an insulating surface, a gate insulating film 1623 over thegate electrode 1622, conductive films 1625 and 1626 over the gateinsulating film 1623, and a semiconductor film 1624 that overlaps thegate electrode 1622 with the gate insulating film 1623 placedtherebetween and is formed over the conductive films 1625 and 1626. Aninsulating film 1627 formed over the conductive films 1625 and 1626 andthe semiconductor film 1624 may be considered as a component of thetransistor.

The transistor in FIG. 13C may also include a backgate electrode thatoverlaps the semiconductor film 1624 with the insulating film 1627placed therebetween.

A transistor illustrated in FIG. 13D is a top-gate bottom-contacttransistor.

The transistor illustrated in FIG. 13D includes conductive films 1645and 1646 formed on an insulating surface, a semiconductor film 1644formed over insulating surface and the conductive films 1645 and 1646, agate insulating film 1643 formed over the semiconductor film 1644 andthe conductive films 1645 and 1646, and a gate electrode 1642 thatoverlaps the semiconductor film 1644 with the gate insulating film 1643placed therebetween. An insulating film 1647 formed over the gateelectrode 1642 may be considered as a component of the transistor.

The transistor in this embodiment can be used as any of the transistorsincluded in the basic circuits, the sequential circuits, and the shiftregister circuit described in Embodiments 1 to 3. It should beparticularly noted that the off-state of the transistor including anoxide semiconductor is low in this embodiment. Consequently, with theuse of such a transistor in the basic circuits, the sequential circuits,and the shift register circuit described in Embodiments 1 to 3, chargeleaked from the node N1, the node N2, and the like can be decreased.This leads to lowering of the driving frequency.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 5

FIG. 14 illustrates an example of a panel that corresponds to oneembodiment of a display device. The panel illustrated in FIG. 14includes a substrate 700 and a pixel portion 701, a signal line drivercircuit 702, a scan line driver circuit 703, and a terminal 704 that areprovided over the substrate 700.

The pixel portion 701 includes a plurality of pixels. Each pixelincludes a display element and at least one transistor for controllingthe operation of the display element. The scan line driver circuit 703selects a pixel included in the pixel portion 701 by controlling supplyof potentials to scan lines connected to the pixels. The signal linedriver circuit 702 controls supply of an image signal to the pixelselected by the scan line driver circuit 703.

At least one of the signal line driver circuit 702 and the scan linedriver circuit 703 can include the basic circuit, the sequentialcircuit, or the shift register circuit described in Embodiments 1 to 3.With such a structure, the effects described in Embodiments 1 to 3 canbe achieved, and the size of the pixel portion 701 can be increased.Moreover, a larger number of pixels can be provided in the pixel portion701.

As the display element, a liquid crystal element or a light-emittingelement can be used, for example.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

Embodiment 6

The semiconductor device according to one embodiment of the presentinvention can be used for display devices, personal computers, and imagereproducing devices provided with recording media (typically, devicesthat reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Other examples of electronic devices that can include the semiconductordevice according to one embodiment of the present invention are mobilephones, game consoles including portable game consoles, personal digitalassistants, e-book readers, cameras such as video cameras and digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio systems and digitalaudio players), copiers, facsimiles, printers, multifunction printers,automated teller machines (ATM), and vending machines. FIGS. 15A to 15Eillustrate specific examples of these electronic devices.

FIG. 15A illustrates a portable game console including a housing 5001, ahousing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, a speaker 5006, an operation key 5007, a stylus 5008,and the like. By using the semiconductor device according to oneembodiment of the present invention in a driver circuit of a portablegame console, a low-power portable game console that operates stably canbe provided. Note that although the portable game console in FIG. 15Aincludes the two display portions 5003 and 5004, the number of displayportions included in the portable game console is not limited to two.

FIG. 15B illustrates a display device including a housing 5201, adisplay portion 5202, a support base 5203, and the like. By using thesemiconductor device according to one embodiment of the presentinvention in a driver circuit of a display device, a low-power displaydevice that operates stably can be provided. Note that a display deviceincludes, in its category, any display device for displayinginformation, such as display devices for personal computers, TVbroadcast reception, and advertisement.

FIG. 15C illustrates a laptop computer including a housing 5401, adisplay portion 5402, a keyboard 5403, a pointing device 5404, and thelike. By using the semiconductor device according to one embodiment ofthe present invention in a driver circuit of a laptop computer, alow-power laptop computer that operates stably can be provided.

FIG. 15D illustrates a personal digital assistant including a firsthousing 5601, a second housing 5602, a first display portion 5603, asecond display portion 5604, a joint 5605, an operation key 5606, andthe like. The first display portion 5603 is provided in the firsthousing 5601, and the second display portion 5604 is provided in thesecond housing 5602. The first housing 5601 and the second housing 5602are connected to each other with the joint 5605, and the angle betweenthe first housing 5601 and the second housing 5602 can be changed withthe joint 5605. Images on the first display portion 5603 may be switchedin accordance with the angle at the joint 5605 between the first housing5601 and the second housing 5602. By using the semiconductor deviceaccording to one embodiment of the present invention in a driver circuitof a personal digital assistant, a low-power personal digital assistantthat operates stably can be provided.

FIG. 15E illustrates a mobile phone including a housing 5801, a displayportion 5802, an audio input portion 5803, an audio output portion 5804,operation keys 5805, a light-receiving portion 5806, and the like. Lightreceived in the light-receiving portion 5806 is converted intoelectrical signals, whereby external images can be loaded. By using thesemiconductor device according to one embodiment of the presentinvention in a driver circuit of a mobile phone, a low-power mobilephone that operates stably can be provided.

This embodiment can be implemented in combination with any otherembodiment as appropriate.

This application is based on Japanese Patent Applications serial No.2011-265799 filed with Japan Patent Office on Dec. 5, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor comprising a source and a drain one of which is supplied witha first signal; a second transistor comprising a source and a drain oneof which is supplied with a first potential, and a gate supplied with asecond signal; and a capacitor comprising a first electrode electricallyconnected to the other of the source and the drain of the firsttransistor, and a second electrode electrically connected to the otherof the source and the drain of the second transistor, wherein in a firstperiod, the first signal is low and the second signal is high, andwherein in a second period, the first signal is high and the secondsignal is either low or high.
 2. The semiconductor device according toclaim 1, further comprising: a fourth transistor comprising a source anda drain one of which is supplied with the first potential and the otherof which is electrically connected to the other of the source and thedrain of the first transistor, and a gate electrically connected to theother of the source and the drain of the second transistor; a fifthtransistor comprising a source and a drain one of which is supplied withthe first potential and the other of which is electrically connected toa gate of the first transistor, and a gate electrically connected to theother of the source and the drain of the second transistor; and a sixthtransistor comprising a source and a drain one of which is supplied witha second potential and the other of which is electrically connected tothe other of the source and the drain of the second transistor, and agate supplied with a third signal.
 3. The semiconductor device accordingto claim 1, wherein a ratio of a channel width to a channel length ofthe first transistor is higher than a ratio of a channel width to achannel length of the second transistor.
 4. An electronic devicecomprising the semiconductor device according to claim
 1. 5. Asemiconductor device comprising: a first transistor comprising a sourceand a drain one of which is supplied with a first signal; a secondtransistor comprising a source and a drain one of which is supplied witha first potential, and a gate supplied with a second signal; a thirdtransistor comprising a source and a drain one of which is supplied withthe first signal, and a gate electrically connected to a gate of thefirst transistor; and a capacitor comprising a first electrodeelectrically connected to the other of the source and the drain of thefirst transistor, and a second electrode electrically connected to theother of the source and the drain of the second transistor, wherein in afirst period, the first signal is low and the second signal is high, andwherein in a second period, the first signal is high and the secondsignal is either low or high.
 6. The semiconductor device according toclaim 5, further comprising: a fourth transistor comprising a source anda drain one of which is supplied with the first potential and the otherof which is electrically connected to the other of the source and thedrain of the first transistor, and a gate electrically connected to theother of the source and the drain of the second transistor; a fifthtransistor comprising a source and a drain one of which is supplied withthe first potential and the other of which is electrically connected tothe gate of the first transistor, and a gate electrically connected tothe other of the source and the drain of the second transistor; and asixth transistor comprising a source and a drain one of which issupplied with a second potential and the other of which is electricallyconnected to the other of the source and the drain of the secondtransistor, and a gate supplied with a third signal.
 7. Thesemiconductor device according to claim 5, wherein a ratio of a channelwidth to a channel length of the first transistor is higher than a ratioof a channel width to a channel length of the second transistor.
 8. Anelectronic device comprising the semiconductor device according to claim5.